Traversing relationships based on lookup strings

[Francis Devereux]

Hi,

Is there a Django function that takes a model instance and a lookup string, and returns the value of the related field?

For example:

the_function_i_am_looking_for(trip, 'booking__client__name')

would return the same as:

trip.booking.client.name

Thanks,

Francis

[James Schneider]

Nothing in Django specifically that I'm aware of (although that's not saying much), but you can probably do something pretty slick with the operator library and attrgetter:

https://docs.python.org/2/library/operator.html#operator.attrgetter

It would probably look something like this (untested):

# define a function to pull the nested attribute

# assume this is in myapp/utils.py

def get_item_attr(obj_instance, search_string):

    dotted_string = search_string.replace('__', '.')

    instance_attr = operator.attrgetter(dotted_string)

    instance_attr_value = instance_attr(obj_instance)

    return item_attr_value

# later, in a view, maybe...

from myapp.utils import get_item_attr

booking_client_name = get_item_attr(trip, 'booking__client__name')

Obviously this is naive and you'll need to check for failures at various points and raise some exceptions if attributes can't be found/accessed, but I think this may point you in the right direction.

Never tried this on my own so YMMV. Note that you'll incur multiple DB hits using a function like this (for the FK traversals), so be careful if you plan to run such a function several times for a single request.

-James

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[James Schneider]

Whoops, I meant to swap out 'item' for 'instance' to make it a bit more readable, but got myself distracted and forgot to finish and hit send.

So, s/item/instance/...

HTH,

-James

[Francis Devereux]

On 2 Mar 2015, at 22:11, James Schneider <jrschn...@gmail.com> wrote:

Nothing in Django specifically that I'm aware of (although that's not saying much), but you can probably do something pretty slick with the operator library and attrgetter:

https://docs.python.org/2/library/operator.html#operator.attrgetter

Thanks - I thought that I might have to roll my own solution if there wasn't anything in Django, but operator.attrgetter looks like it'll make rolling my own easier.

Francis